Circuit Switching vs Packet Switching

Understanding the fundamental difference between circuit switching and packet switching is crucial to understanding how voice and data work in telecom networks.

Overview

Circuit Switching

Concept

Circuit switching establishes a dedicated communication path between two parties for the entire duration of the communication session.

How It Works

Characteristics

1. Dedicated Path

A physical (or virtual) circuit is reserved
Same path for entire call duration
No other users can use these resources

2. Three Phases

Setup:

Establish end-to-end path
Reserve bandwidth
Can take several seconds

Data Transfer:

Continuous transmission
Guaranteed bandwidth
Low latency
Fixed delay

Teardown:

Release all resources
Circuit becomes available for others

3. Guaranteed Quality of Service (QoS)

Fixed bandwidth for call duration
Consistent latency
No congestion during call
Predictable performance

4. Inefficient for Bursty Traffic

Resources reserved even during silence
Cannot be shared
Wastes bandwidth

Example: Traditional Phone Call (2G/3G)

Voice Call Properties:

64 Kbps per direction (PCM encoding)
Circuit reserved for entire call
Silence also consumes bandwidth
Immediate teardown when call ends

Advantages ✅

Guaranteed Bandwidth: Fixed capacity during session
Consistent Quality: No packet loss or jitter
Low Latency: Direct path, no routing delays
Simple: Straightforward implementation
Predictable: Always same performance

Disadvantages ❌

Inefficient: Wastes bandwidth during silence
Expensive: Dedicated resources cost more
Poor Scalability: Limited by physical circuits
Setup Delay: Takes time to establish circuit
Inflexible: Cannot dynamically adjust bandwidth
Blocking: Call fails if no circuits available

Used In:

Traditional PSTN (landline phones)
2G GSM voice calls
3G UMTS voice calls
ISDN connections

Packet Switching

Concept

Packet switching breaks data into packets that are independently routed through the network using shared resources.

How It Works

Packet Structure

Characteristics

1. Store and Forward

Each router receives complete packet
Stores it in buffer
Examines header
Forwards to next hop
Continues until destination

2. Shared Resources

Multiple users share same link
Statistical multiplexing
More efficient use of bandwidth

3. No Dedicated Path

Each packet routed independently
Packets may take different routes
Can arrive out of order
Receiver must reorder

4. Variable Delay

Queuing delay at routers
Different paths = different delays
Jitter: Variation in packet arrival times

Types of Packet Switching

1. Datagram (Connectionless)

Characteristics:

No connection setup
Each packet routed independently
Packets may arrive out of order
Used in IP networks (Internet)

Example: Internet (IP protocol)

2. Virtual Circuit (Connection-Oriented)

Characteristics:

Connection setup phase (like circuit switching)
Logical path established
All packets follow same route
Packets arrive in order
Connection teardown

Examples:

ATM (Asynchronous Transfer Mode)
MPLS (Multi-Protocol Label Switching)
Frame Relay

Advantages ✅

Efficient: Bandwidth shared among users
Flexible: Dynamic bandwidth allocation
Scalable: Easier to add more users
Robust: Alternate routes if link fails
Cost-effective: Better resource utilization
No Blocking: Almost always accepts data

Disadvantages ❌

Variable Delay: Inconsistent latency (jitter)
Packet Loss: Congestion can drop packets
Out of Order: Packets may arrive scrambled
Complex: Requires sophisticated routing
No Guarantees: Best-effort delivery (unless QoS)
Overhead: Header in every packet

Used In:

Internet (IP networks)
GPRS/EDGE (2.5G data)
3G data (HSPA)
4G/LTE (all traffic including voice)
VoLTE (Voice over LTE)

Comparison Table

Feature	Circuit Switching	Packet Switching
Path	Dedicated	Shared
Setup	Required	Optional (datagram)
Bandwidth	Fixed, reserved	Dynamic, shared
Delay	Fixed, low	Variable (jitter)
Resource Utilization	Poor (wasted during silence)	Excellent
Reliability	High (guaranteed delivery)	Lower (best effort)
Overhead	Setup time only	Per-packet headers
Congestion	Blocking (call rejected)	Delay/packet loss
Order	Always in order	May be out of order
Examples	PSTN, 2G/3G voice	Internet, VoLTE
Charging	By time	By volume
Scalability	Limited	Excellent
Cost	Higher	Lower

Voice Transmission Comparison

Circuit Switched Voice (2G/3G)

Characteristics:

64 Kbps PCM encoding
Continuous stream
Circuit reserved for entire call
Wastes bandwidth during silence

Packet Switched Voice (VoLTE)

Characteristics:

Voice encoded into packets (AMR codec)
Each packet: ~20ms of audio
Silence suppression: No packets during silence
Shares bandwidth with data
More efficient

Real-World Example: Phone Call

Scenario: 10-minute call where people speak 50% of the time

Circuit Switched (2G/3G)

Setup: 3 seconds
Call Duration: 10 minutes (600 seconds)
Circuit: 64 Kbps reserved for 600 seconds
Actual voice: 300 seconds (50%)
Silence: 300 seconds (50%)

Bandwidth Used: 64 Kbps × 600s = 4,800 Kb
Efficiency: 50% (half wasted on silence)

Packet Switched (VoLTE)

Setup: 1.5 seconds (SIP)
Call Duration: 10 minutes (600 seconds)
Packets only during speech: 300 seconds
Silence: No packets sent

Bandwidth Used: 64 Kbps × 300s = 2,400 Kb
Efficiency: Nearly 100% (silence suppression)
Bandwidth Saved: 50%

Plus: Remaining bandwidth available for data!

Network Evolution: Circuit to Packet

Why the Shift?

Efficiency: Better bandwidth utilization
Convergence: Single network for voice and data
Cost: Cheaper infrastructure
Features: Easier to add new services
Internet: Native IP support
Quality: HD voice possible

Hybrid: CSFB (Circuit Switched Fallback)

During early LTE deployment, many networks didn't have VoLTE. Solution: CSFB

Drawbacks:

Call setup delay (4-5 seconds)
Drops to 3G (slower data)
Poor user experience

Solution: VoLTE (all packet switched!)

Quality of Service (QoS) in Packet Networks

To make packet switching work for voice, QoS is critical.

QoS Mechanisms

LTE QoS: QCI (QoS Class Identifier)

QCI	Priority	Packet Delay	Packet Loss	Use Case
1	2	100 ms	10⁻²	VoLTE
2	4	150 ms	10⁻³	Video call
3	3	50 ms	10⁻³	Real-time gaming
5	5	100 ms	10⁻⁶	IMS signaling
9	9	300 ms	10⁻⁶	Web browsing
8	8	300 ms	10⁻⁶	Video streaming

VoLTE gets QCI=1: Highest priority, low latency!

Summary